Electromagnetic energy transducer

ABSTRACT

An electromagnetic energy transducer configured to convert mechanical energy into electrical energy comprises two magnetic elements including a permanent magnetic element and a soft-magnetic element and an electrical coil. The permanent magnetic element and the soft-magnetic element are arranged to form a magnetic circuit and one of the two magnetic elements is movable in relation to the other of the two magnetic elements. The electrical coil surrounds a part of the soft magnetic element. The movable magnetic element is held in a first position by a spring force and moved into a second position by applying an external mechanical force exceeding the spring force, and at the first position the magnetic flux within the soft magnetic element is different than the flux at the second position.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/757488, filed Apr. 9, 2010, which is a Continuation of U.S.application Ser. No. 11/245,615, filed Oct. 6, 2005, which is acontinuation of International Application No. PCT/DE2004/000681, filedon Apr. 1, 2004, which claims priority from German application no. 10315 765.4, filed Apr. 7, 2003, the contents of which are herebyincorporated by reference.

FIELD OF THE INVENTION

The invention relates to an electromagnetic energy transducer having apermanent magnet, a soft-magnetic element and an electrical coil.

BACKGROUND OF THE INVENTION

Many different embodiments of electromagnetic energy transducers areknown. Generators or electric motors may be mentioned by way of examplein this context. In principle, every electromagnetic energy transduceris suitable for conversion of mechanical energy to electrical energy, orof electrical energy to mechanical energy. These electromagnetic energytransducers are generally designed such that mechanical energy in theform of a rotary movement is converted to electrical energy by means ofthe electromechanical energy transducer.

Electromagnetic energy transducers such as these all have the commontask of supplying power to electrical loads continuously and over alengthy time period, or of converting electrical energy to mechanicalenergy continuously over a lengthy time period. In this case, varioussizes and embodiments are known, depending on the requirement. In thiscase, it can be assumed as a fundamental rule that the conversion ofelectrical energy to mechanical energy or of mechanical energy toelectrical energy can be carried out with higher efficiencies, that isto say with lower losses, as the size of the electromagnetic transducerincreases. Conversely, this means that, the smaller an electromagnetictransducer is intended to be, the greater the proportions of the powerlosses which occur in the electromagnetic transducer become, or thelower the efficiency becomes. This is particularly important forelectromagnetic transducers which supply electrical power toautonomous-power systems, for example radio switches or radiotransmitters which do not have a battery-powered or wire-based powersupply. Electromagnetic transducers which are used in this way mustprovide sufficient power for operation of a system such as this at thetime at which the power is required. Since the physical conditions forsystems such as these are often highly restricted so that it isnecessary to use very small electromagnetic transducers, it can also beassumed that the efficiencies will be very low, on the basis of thefundamental rule mentioned above.

SUMMARY OF THE INVENTION

One object of the invention is to provide an electromagnetic energytransducer which represents a power supply for miniaturizedautonomous-power systems such as, in particular, radio switches, whichpower supply provides sufficient power for operation of the system, forexample of the radio switch, at the time at which the power is required.

This and other objects are attained in accordance with one aspect of thepresent invention directed to an electromagnetic energy transducercomprising a permanent magnet, a soft-magnetic element, an electricalcoil and stop points. The electrical coil surrounds a part of themagnetic circuit, wherein the permanent magnet and the soft-magneticelement are arranged to form a magnetic circuit with a first fluxdirection. At least one of the soft-magnetic element and the permanentmagnet is mounted for rotary movement about an axis with respect to theother. End points of the rotary movement are formed by the stop points.

Advantageous features of this electromagnetic transducer include thatthe number of moving elements is small, and the movement is likewiseintrinsically somewhat small, since it describes only a predeterminedmovement distance in one direction in each case, specifically from onestop point to another. There are thus no rotation-dependent frictionlosses resulting, for example, from roller, ball or journal bearings,which have to withstand high rotation speeds over a long time. Thenumber of components for an energy transducer such as this is alsoreasonably small, since, in principle, the three components mentionedabove, specifically the permanent magnet, the soft-magnetic element andthe electrical coil and the connecting axis, describe all of thenecessary components. There is no need for complex power taps andtransfer systems such as sliding contacts, contact commutators etc., andthe friction losses and wear phenomena associated with them therefore donot occur for this reason in the case of the electromagnetic transduceraccording to this aspect of the invention.

The axis by means of which the permanent magnet and the soft-magneticelement are mounted such that they can rotate with respect to oneanother causes a reversal of the magnetic flux through the electricalcoil in conjunction with the stop points. The stop points allow themagnetic flux to be reversed as quickly as possible, in particularsuddenly. This characteristic takes account of the law that the rate ofchange of the magnetic flux is directionally proportional to theelectrical energy converted. The coil, which is in this case preferablyarranged around the soft-magnetic element of the magnetic circuit, isthus provided with a very large amount of induction. This isparticularly advantageous because this high induction is changed not byclosing or opening the magnetic circuit, but is changed by twice theamount by changing the direction of the magnetic circuit. The rapidmagnetic flux change which is produced in this way leads to a voltagebeing induced briefly in the coil, and the electrical energy which isproduced in this way can be rectified by means of rectifiers, preferablysemiconductor metal contact diodes, and, after temporary storage, acapacitor, for example, can be used for brief operation of a radioswitch or radio sensor.

The rotary movement is initiated, for example, by introduction of anadditional force, for example by a user. If, by way of example, thesoft-magnetic element is located with one end at the north pole of thepermanent magnet and with the other end at the south pole of thepermanent magnet, then this position is held by the magnetic force. Thismagnetic holding force must be overcome for operation of theelectromagnetic energy transducer. This is done by introduction of anadditional external force in the opposite rotation direction to that ofthe magnetic holding force. If the force that is introduced is greaterthan the magnetic holding force, a rotary movement starts suddenly inthe direction of the introduced force. This on the one hand interruptsthe existing magnetic circuit and on the other hand results in it beingclosed again on reaching the stop points of the magnetic circuit in theopposite direction. If the contact surfaces at the stop points areformed directly by the permanent magnet and the soft-magnetic element,then the magnetic flux is not opposed by any further resistance as wouldbe formed, by way of example, by an air gap so that the magnetic fluxcan be changed to the maximum extent in an extremely short time.

Two limit positions of the rotary movement are advantageously formed,between which the rotary movement takes place in the form of a rockingmovement. Both limit positions represent stable limit positions of thepossible rotary movement, assisted by the magnetic latching forces.Whenever an external force is introduced in the opposite direction tothat of the magnetic latching force, a sudden snapping action isinitiated in this way, and electrical energy is produced. In order toadvantageously influence the timing, it is worthwhile to keep the angledescribed by the rotary movement small, so that the time consumed tocarry out the movement is as short as possible. The critical factor withrespect to the production of the amount of electrical energy is not themovement distance of the rotary movement but essentially the rate ofchange of the magnetic flux which is increased, in particular, by thereversal of the magnetic flux.

In one advantageous embodiment, one rest position of the rotary movementis supported by a spring element, to be precise in such a way that thesecond rest position snaps back again to the first rest positionimmediately after being reached by the introduction of external force,since the second rest position is kept unstable by the force of thespring element. This means that the operation of the energy transducertakes place by the introduction of external force against the magneticlatching force and against the spring force such that, when the latchingforce is overcome, this results in a sudden snapping action to thesecond rest position. Sudden snapping back to the first rest position islikewise achieved, driven by the spring force. The electrical energywhich is produced in this way is in consequence twice the amount thatwould be produced by a simple snapping action from the first restposition to the second rest position.

This is a question of the dimensioning of the spring element as well asa question of the particular application for which the likewiseadvantageous embodiments which are described briefly in the followingtext are used.

The spring element can thus be dimensioned together with the magneticelements in such a way that a rotary movement from the first restposition or the second rest position is assisted by the force of thespring element, so that less external force need be applied in order tooperate the electromagnetic transducer. This means that the springelement is not designed to be sufficiently strong to reverse thisprocess again and thus to produce twice the energy.

On the other hand the spring element could, for example, be designedsuch that the spring force is balanced by the magnetic forces in amid-position of the rotary movement, thus allowing energy to be producedin the form of a momentary-contact element in both directions. Theamount of energy is thus, of course, less than in one of the embodimentsmentioned above, but, in the end, this is once again a question of thedimensions of the permanent magnet, soft-magnetic element and coil inthis case.

The coil is advantageously arranged around the soft-magnetic element,such that the soft-magnetic element forms a coil core, since this iswhere the greatest flux change can be achieved by the permanent magnet.An arrangement of the coil around the permanent magnet would in no wayachieve this effect.

Since the two elements, the permanent magnet and the soft-magneticelement, are mounted such that they can rotate with respect to oneanother, it depends in the end on the application which of the twoelements is firmly connected to a third element, for example to ahousing. The spring is also supported with respect to this housing orthird element, in order to exert its force. A spring can also to acertain extent be arranged in the form of a spiral spring or can bearranged on the two elements in such a way that the force acts directlybetween the two elements. The critical factor for the purposes of theinvention is that the coil results in a change, in particular areversal, of the magnetic flux as a result of the rotary movement, whichshould be as short as possible. The sudden nature of this change is acritical factor for the electrical energy that is obtained.

By way of example, iron ferrite, a nickel/iron alloy or else so-calledelectrical or transformer laminates is or are suitable for use as thesoft-magnetic material. In the exemplary embodiments mentioned above,the use of an electromagnetic transducer such as this for radio switchesor radio sensors, but in particular for radio switches, is particularlyadvantageous since an electromagnetic transducer such as this can beused to initiate the rotary movement at the time of operation of theradio switch. The rotary movement results in a voltage being induced,which is used to supply power to the radio switch.

According to another aspect of the present invention, an electromagneticenergy transducer configured to convert mechanical energy intoelectrical energy comprises two magnetic elements including a permanentmagnetic element and a soft-magnetic element and an electrical coil. Thepermanent magnetic element and the soft-magnetic element are arranged toform a magnetic circuit, and one of the two magnetic elements is movablein relation to the other of the two magnetic elements. The electricalcoil surrounds a part of the soft magnetic element. The movable magneticelement is held in a first position by a spring force and moved into asecond position by applying an external mechanical force exceeding thespring force, and at the first position the magnetic flux within thesoft magnetic element is different than the flux at the second position.

At the first position at least in one of direction or strength, themagnetic flux within the soft magnetic element is different than theflux at the second position. The movable magnetic element moves from thesecond position back into the first position when the externalmechanical force no longer exceeds the spring force.

At least one of the soft-magnetic element and the permanent magneticelement is held in a first position by a magnetic holding force. A firstand a second limit position are formed for the movement of the movablemagnetic element. The first limit position is defined by one set of stoppoints, and the second limit position is defined by another set of stoppoints. A stable first or second rest position is formed by magneticlatching forces in the first or second limit position. The movement isthrough an angle of less than 90°.

The soft-magnetic material is iron, ferrite or a nickel/iron alloy, oris formed from electrical or transformer laminates. The electromagnetictransducer supplies electrical power to an autonomous-device and mayfurther comprise an autonomous-power switch comprising a radio switch ora radio signal transmitter and/or a contact portion configured toreceive the applied external mechanical force.

The soft magnetic element comprises a first end and a second end and thepermanent magnet comprises a north pole and a south pole. The first endcontacts the north pole and the second end contacts the south pole toform the magnetic circuit with the first flux direction, and the firstend contacts the south pole and the second end contacts the north poleto form the magnetic circuit with the second flux direction. A first endof the soft magnetic element contacts a north pole of the permanentmagnet and a second end of the soft magnetic element contacts a southpole of the permanent magnet in the first rest position, and the firstend of the soft magnetic element contacts the south pole of thepermanent magnet and the second end of the soft magnetic elementcontacts the north pole of the permanent magnet in the second restposition.

According to still another aspect of the present invention, a method forconverting mechanical energy into electrical energy uses a device thatcomprises two magnetic elements including a permanent magnetic elementand a soft-magnetic element and an electrical coil. The permanentmagnetic element and the soft-magnetic element are arranged to form amagnetic circuit with a magnetic flux within the soft magnetic element.One of the two magnetic elements is movable in relation to the other ofthe two magnetic elements. The electrical coil surrounds a part of thesoft magnetic element. The method comprises applying a spring force tothe movable magnetic element for holding the moveable magnetic elementin a first position. An external mechanical force that exceeds thespring force is applied against the spring force for moving the movablemagnetic element into a second position so that the magnetic flux withinthe soft magnetic element is changed.

At least the direction or the strength of the magnetic flux within thesoft magnetic element is changed. The external mechanical force isreduced to a level that no longer exceeds the spring force.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an electromagnetic transducer in a first rest position.

FIG. 2 shows an electromagnetic transducer in a second rest position.

FIG. 3 shows another embodiment of an electromagnetic transducer.

FIG. 4 shows yet another embodiment of an electromagnetic transducer.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an electromagnetic transducer according to an embodiment ofthe invention in which a permanent magnet 1 is formed, with a firstpermanent-magnet layer 2 and a second permanent-magnet layer 3. An axis4 forms the rotation axis, about which the rotary movement takes place.In this embodiment, a moving element 5 is mounted by means of the axis 4such that it can rotate with respect to the permanent magnet 1. Suchmounting arrangements are well known to a person with ordinary skill inthe art. Any such mounting can be used.

An electrical coil is arranged at least partially around the movingelement 5 and an electric current or voltage is induced in it when themagnetic flux changes. The spring force of a spring element 7counteracts any externally introduced force. The externally introducedforce then presses downward against the spring force and, above aspecific magnitude, thus overcomes the magnetic holding forces, so thatthe externally introduced force initiates a rotary movement, whichresults in a sudden movement to the second rest position.

This second rest position is illustrated in FIG. 2. Stop points 8 a, 8b, 8 c and 8 d are formed on the permanent magnet 1. For example, stoppoints 8 a and 8 d are south poles stop points 8 b and 8 c are northpoles. The rotary movement is possible only within these stop points.Abrupt striking of the moving element 5 against the stop points 8 a to 8d thus results in an abrupt, in fact sudden, change in the magnetic fluxdirection in the moving element 5. In the exemplary embodiment, thismoving element is advantageously in the form of a soft-magnetic element9. This soft-magnetic embodiment allows rapid reorientation of themagnetic flux direction in the moving element. The spring force of thespring element 7 can be adjusted such that the second rest positionillustrated in FIG. 2 is an unstable position, with the spring elementdriving the moving element 5 and/or the soft-magnetic element 9 back tothe first rest position again, as illustrated in FIG. 1. This is anexample of the most energy-efficient exemplary embodiment.

FIG. 3 shows another embodiment of the energy transducer, with adifferent physical embodiment of the permanent magnet and of thesoft-magnetic element 9. In this case, the permanent magnet 1 preferablyalso has a layer-like formation, thus forming a first permanent-magnetlayer 2 and a second permanent-magnet layer 3. The axis 4 in this caseonce again represents the rotation axis about which the two elements aremounted such that they can rotate with respect to one another, inparticular, however, with the permanent magnet being able to rotatewithin the degrees of freedom or angular degrees formed by the stoppoints 8 a to 8 d on soft-magnetic element 9. In this embodiment, themoving element 5 is in the form of a permanent magnet and thesoft-magnetic element 9 is fixed to a stationary third element such as,for example, a housing. This is particularly advantageous for thecontact points for the coil 6, since this embodiment then has a longerlife because the electrical contact points of the coil 6 arranged onsoft-magnetic element 9 are exposed to less mechanical wear.

As shown in FIG. 4, the spring element 7 could be designed such that thespring force is balanced by the magnetic forces in a mid-position of therotary movement of the moving element 5. Energy then can be produced inthe form of a push-button in both directions. The rotary movement thencan be initiated in both possible directions.

In principle, the above-discussed embodiments are particularly suitablefor use with radio switches which do not have their own power supply,but obtain the power supply for a radio signal from a switching pulse.

An energy transducer as is disclosed above makes it possible to carryout an energy conversion process when force is introduced andadditionally when the force is removed. Furthermore, a minimum amount ofenergy can be converted independently of the rate of operation since,even if the force is introduced slowly, the time at which the holdingforce is exceeded results in a sudden snapping-action to the other restposition. This makes it possible to produce a toggle switch ormomentary-contact switch in a simple manner. Since the polarity of theelectrical power that is produced also differs depending on thedirection in which the magnetic flux is changed, this information caninitially be used in a radio switch in order, for example, to be able tomap two different functions of a radio switch separately from oneanother.

When an externally introduced force is exerted on this moving part,nothing happens until the holding force produced by the permanent magnetin the moving part is exceeded. The moving part then moves relativelyquickly to the second stable position, in which it remains fixed againby the latching forces of the permanent magnet. This snapping actiontakes place, even if the externally introduced force is increasedslowly, at a minimum speed which can be varied by the design of themagnetic circuit. This ensures that sufficient energy for operation ofthe radio transmitter/radio switch is converted even in the event ofslow application of the externally introduced force.

The magnetic flux through that part of the magnetic circuit 5 aroundwhich the coil 6 is wound (in the first embodiment) changes itsdirection on snapping around an axis 4 or a toggle point. This ispreferable to designs in which the magnetic flux is just interrupted orclosed, since the flux change is in this case twice as great.

The rapid magnetic flux change leads to voltage being induced briefly inthe coil, and the electrical energy produced in this way can berectified by means of rectifiers, preferably semiconductor metal contactdiodes, and, after temporary storage in a capacitor, can be used tobriefly operate a radio switch or radio sensor. It is also feasible touse a plurality of coils and to dispense with the rectification of theenergy, which results in high losses, particularly in the case of lowvoltages.

The spring element 7 is used to form a momentary-contact switch from achangeover switch with two fixed positions. For this purpose, the springforce is of such a magnitude that, after the snapping action, the springforce is sufficient in order to quickly move the moving part back to theinitial position again against the holding force as soon as theexternally introduced force becomes sufficiently small. This allows aphysically simple implementation of a monostable switch. In this case,according to this embodiment of the invention, energy is produced notonly when the switch is pushed, but also when it is released.

The polarity of the voltage that is produced changes with the way ofoperation. According to the invention, this polarity can be measured bythe connected electronics, and the information contained in it about thedirection of the state change can be transmitted with the radio signalthat is to be transmitted.

The mechanism which acts on the moving part of the energy transducer hasat least one operating element, for example a surface of a push-button.According to the invention, a plurality of operating devices can alsoact in the same way on one energy transducer if the radio switch isintended to be implemented with a plurality of channels. In this case, asufficient number of sensors, for example one sensor per operatingdevice, are used to ensure that the respectively activated operatingdevice is determined by the connected electronics. The information isthen included in the radio message to be transmitted.

The radio transmitter which can be operated with the energy transducerhas at least one operating device, which acts mechanically on the energytransducer to apply the externally introduced force. If it has aplurality of operating devices, these all act on the single energytransducer and, in addition to a suitable number of sensors, are usedfor detection of the respectively activated operating device. Theinformation about the identity of the operating field can thus beincluded in the radio signal to be transmitted.

The information about the way of operation (pressing or releasing in thecase of momentary-contact switches or pressing on different operatingservices in the case of toggle switches) can be sensed from the polarityof the voltage that is produced, and can likewise be transmitted bymeans of the radio signal.

The scope of protection of the invention is not limited to the examplesgiven hereinabove. The invention is embodied in each novelcharacteristic and each combination of characteristics, which includesevery combination of any features which are stated in the claims, evenif this combination of features is not explicitly stated in the claims.

1. An electromagnetic energy transducer configured to convert mechanicalenergy into electrical energy, comprising: two magnetic elementsincluding a permanent magnetic element and a soft-magnetic element; andan electrical coil, wherein said permanent magnetic element and saidsoft-magnetic element are arranged to form a magnetic circuit, and oneof the two magnetic elements is movable in relation to the other of thetwo magnetic elements, wherein said electrical coil surrounds a part ofsaid soft magnetic element, wherein the movable magnetic element is heldin a first position by a magnetic force of the magnetic circuit andmoved into a second position by applying an external mechanical forceexceeding the magnetic force, wherein at the first position the magneticflux within the soft magnetic element is different than the flux at thesecond position, wherein a first and a second limit position are formedfor the movement of the movable magnetic element, wherein said firstlimit position is defined by a first set of stop points, and said secondlimit position is defined by a second set of stop points, wherein thepermanent magnetic element is formed as a planar integral unitcomprising: a first layer having a first polarity, a second layer havinga polarity opposite from that of the first polarity, a first set ofprotrusions corresponding to the first set of stop points, and a secondset of protrusions corresponding to the second set of stop points;wherein each set of protrusions includes at least on protrusioncorresponding to the first layer and at least one protrusioncorresponding to the second layer.
 2. The electromagnetic energytransducer as claimed in claim 1, wherein at the first position at leastin one of direction or strength, the magnetic flux within the softmagnetic element is different than the flux at the second position. 3.The electromagnetic energy transducer as claimed in claim 1, wherein themovable magnetic element moves from the second position back into thefirst position when the external mechanical force no longer exceeds themagnetic force.
 4. The electromagnetic energy transducer as claimed inclaim 1, wherein at least one of the soft-magnetic element and thepermanent magnetic element is held in a first position by a magneticholding force.
 5. The electromagnetic transducer as claimed in claim 1,wherein a stable first or second rest position is formed by magneticlatching forces in the first or second limit position.
 6. Theelectromagnetic energy transducer as claimed in claim 1, wherein themovement is through an angle of less than 90°.
 7. The electromagneticenergy transducer as claimed in claim 1, wherein the soft-magneticmaterial is iron, ferrite or a nickel/iron alloy, or is formed fromelectrical or transformer laminates.
 8. The electromagnetic transduceras claimed in claim 1, wherein the electromagnetic transducer supplieselectrical power to an autonomous-device.
 9. The electromagnetictransducer as claimed in claim 1, further comprising an autonomous-powerswitch comprising a radio switch or a radio signal transmitter.
 10. Theelectromagnetic transducer as claimed in claim 1, further comprising acontact portion configured to receive the applied external mechanicalforce.
 11. The electromagnetic energy transducer as claimed in claim 1,wherein the soft magnetic element comprises a first end and a second endand the permanent magnet comprises a north pole and a south pole, andwherein the first end contacts the north pole and the second endcontacts the south pole to form the magnetic circuit with the first fluxdirection, and the first end contacts the south pole and the second endcontacts the north pole to form the magnetic circuit with the secondflux direction.
 12. The electromagnetic transducer as claimed in claim11, wherein the first end of the soft magnetic element contacts a northpole of the permanent magnet and the second end of the soft magneticelement contacts a south pole of the permanent magnet in the first limitposition, and the first end of the soft magnetic element contacts thesouth pole of the permanent magnet and the second end of the softmagnetic element contacts the north pole of the permanent magnet in thesecond limit position.
 13. The electromagnetic transducer as claimed inclaim 1, consisting essentially of the two magnetic elements and theelectrical coil.
 14. The electromagnetic transducer as claimed in claim1, further comprising a spring element configured to apply a springforce to at least assist holding the movable magnetic element in thefirst limit position.
 15. The electromagnetic transducer as claimed inclaim 1, consisting essentially of the two magnetic elements, theelectrical coil, and the spring element.
 16. A method for convertingmechanical energy into electrical energy by a device comprising: twomagnetic elements including a permanent magnetic element and asoft-magnetic element; and an electrical coil; wherein said permanentmagnetic element and said soft-magnetic element are arranged to form amagnetic circuit with a magnetic flux within the soft magnetic elementand one of the two magnetic elements is movable in relation to the otherof the two magnetic elements, wherein said electrical coil surrounds apart of said soft magnetic element, wherein a first and a second limitposition are formed for the movement of the movable magnetic element,wherein said first limit position is defined by a first set of stoppoints, and said second limit position is defined by a second set ofstop points, and wherein the permanent magnetic element is formed as aplanar integral unit comprising: a first layer having a first polarity,a second layer having a polarity opposite from that of the firstpolarity, a first set of protrusions corresponding to the first set ofstop points, and a second set of protrusions corresponding to the secondset of stop points; wherein each set of protrusions includes at least onprotrusion corresponding to the first layer and at least one protrusioncorresponding to the second layer, the method comprising the steps:applying a magnetic force of the magnetic circuit to the movablemagnetic element for holding the moveable magnetic element in a firstlimit position; and applying an external mechanical force against themagnetic force and exceeding the magnetic force for moving the movablemagnetic element into a second limit position so that the magnetic fluxwithin the soft magnetic element is changed.
 17. The method forconverting mechanical energy into electrical energy according to claim16, wherein at least the direction or the strength of the magnetic fluxwithin the soft magnetic element is changed.
 18. The method forconverting mechanical energy into electrical energy according claim 16,further comprising the step of reducing the external mechanical force toa level that no longer exceeds the magnetic force.